It is known that resonator-based duplex filters are used in a radio telephone to prevent the sensing of a transmitted signal by the receiver and the sensing of a received signal by the transmitter. The filter has a precisely defined stop band and pass band for each purpose. Since the price of the duplex filters of radio telephones is affected, in addition to other factors, by the number of filters manufactured, this in practice means that a separate construction is not designed for each individual duplex filter version intended for a different radio telephone network; instead, the same construction is used in different versions. However, in different versions the requirements vary, for example with respect to the widths of the stop bands and the pass bands, and thus the basic filter must be modified according to the requirements laid out in the specifications for the particular radio telephone network in use; In this case the basic construction should be applicable also to bandwidths greater than the bandwidths which were the basis for the original electrical design.
In practice the problem is to increase the bandwidth by using the same already existing mechanical construction and without changing the number of resonators. Often the desired new bandwidth is obtained simply by increasing the coupling between the resonator circuits in the duplex filter by some known mechanical method, in which case the number of resonators is thus not increased; it is possible to use as such the original duplex filter construction made up of resonators.
The disadvantage of this bandwidth-increasing prior art technique is that, while the inter-resonator coupling is modified and the bandwidth of the filter is thereby increased, the stop-band attenuation is at the same time reduced. This occurs immediately, without a delay. Sometimes the deteriorated stop-band attenuation can be tolerated, if it still fits the desired requirements, but usually in practice there arises a situation in which excessive deterioration of the stop band will prevent a necessary increasing of the bandwidth with the same mechanical construction.
In radio telephone technology, filters based on surface wave resonators have already been used for some time. They are also called SAW filters (surface acoustic wave filter). They have the advantage not only of a small size but also of precise reproducibility in manufacture. The basic part of a component utilizing the surface wave phenomenon is an interdigital transformer, which is made up of interdigital electrodes in a comb-like arrangement on the surface of a piezoelectric substrate. The inter-electrode electric voltage generates in the substrate acoustic waves which propagate on its surface in a direction which is perpendicular to the interdigital comb electrodes. These surface waves can be received by the interdigital transformer, which reconverts the surface acoustic waves propagating on the substrate surface to an electric voltage signal. As compared with an electromagnetic wave, the propagation speed of a surface acoustic wave on the surface of a piezoelectric substrate is about 1/100,000 times slower. By the surface wave technique it is possible to manufacture many circuits, such as filters, delay lines, resonators, oscillators, etc. One example is the notch filter described in U.S. Pat. No. 4,694,266. Filters using surface wave resonators have also been used in duplex filters, for example in U.S. Pat. No. 4,509,165; a combination technique is used in the filter described in this patent, the front end of the duplex filter, as seen from the antenna, being implemented using dielectric filters and the receiver-side part being implemented using surface wave filters.